(a) Field of the Invention
The present invention relates to a method and apparatus for a switching circuit, and more particularly to a control method and apparatus for pulse-width modulation.
(b) Description of the Prior Art
Electrical energy may have different types of conversions depending on needs, in which the action principle of a direct-current--direct-current converter (dc/dc converter) is that a direct-current is used in a high frequency switching mode to generate high frequency alternating-current, and by means of a high frequency transformer, the voltage is stepped Up or down to attain a desired voltage, which voltage will be converted into a direct current output by a rectifier. The relationship and functions of each portion of the above-mentioned circuit are shown in FIG. 1.
A switching circuit has many different changes in application. FIG.1 shows one of the most typical modes of switching circuits, called the full-bridge mode switching circuit. This circuit structure comprises four transistors, Q1, Q2, Q3, and Q4. The action principle is that when the transistor Q1 conducts, the transistor Q2 does not conduct; and when the transistor Q2 conducts, the transistor Q1 does not conduct. The states of the two transistors Q1 and Q2 are complementary. Also, during a certain interval, both Q1 and Q2 do not conduct. Such a time interval is called deadtime. Therefore, point A has two voltage states. The first one is called the "1" state, i.e., when the transistor Q1 conducts. The other one is called the "0" state, i.e., when the transistor Q2 conducts. Likewise, when the transistor Q3 conducts, the transistor Q4 does not conduct; and when the transistor Q4 conducts, the transistor Q3 does not conduct. The states of the transistors Q3 and Q4 are also complementary. The B point between them also has a deadtime, i.e., when both transistors Q3 and Q4 do not conduct, and the above-described "1" and "0" states. Thus, there are four different voltage states, which are (1,1), (1,0), (0, 1), and (0,0). Actually, in the (1, 1) and (0, 0) voltage states, the voltage outputs to the transformer(T) are the same, zero. Therefore, the output electric power has two nonzero states, (1,0) and (0,1), and two zero states, (1,1) and (0,0).
Based on the above-described principle, conventional dc--dc power conversion control methods fall into two main categories: One is control of the output voltage, and the other is control of the output current. All the prior art in relation to these two main categories of control methods utilize the duty cycles of the so-called non-zero states, (1,0) and (0,1), to proceed with a real-time modulation, wherein the former is called voltage-mode control, and the latter is called current-mode control.
The two types of conventional technologies are described in detail in the following books: (1) "Unitegrated Circuits Data Book and Application Note", Unitrode Integrated Circuits Co. May, 1993 and (2) "Power Electronics; Converters, Applications and Design", Mohan, Undeland, Robbin John Willey Sons, Inc., 1989, and (3) Keith H. Billings, "Handbook of Switch Mode Power Supplies", McGraw-Hill Publishing Company, 1989.
Based on the circuit structure as shown in FIG. 1, the waveforms of the voltage and current of the primary side of tranformer(T), i.e., at the A and B terminals, are respectively shown in FIG. 2 and FIG. 3 individually, wherein in FIG. 3 the areas A, B, C, and D, individually show the current waveform formed by the multiplication of the time and the voltage which the transformer receives. Under ideal circumstances, area A is the same as A+ in FIG. 4, and area B is the same as the inverse phase B+ in FIG. 4. But actually, if the positive and negative waves do not balance, the current of FIG. 3 will have a dc low frequency bias to cause imbalance. Then, the primary side of transformer(T) will generate a large direct current, but the waveform shown in FIG. 4 will not be affected to a great extent. Such an imbalance phenomenon usually causes the transformer to become saturated. To prevent the transformer from being saturated, there must be provided a current detector at the primary side of the transformer to detect and control the current, that is to limit the current so that it is balanced at both sides of the zero current. The are a few control methods which utilize the above-mentioned characteristics, wherein the typical method is peak-current mode control. This method primarily consists of detecting the current as shown in FIG. 3. It is used to detect the absolute value of the current. When the detected absolute value is beyond the default one, then it will be determined whether or not to trigger the transistors Q1 and Q4 to be both off, or to trigger the transistors Q2 and Q3 to be both off. Actually, the current signal generated by means of the above method has many noises which usually cause errors.